Conditional Rewriting in Action

Now we are dealing with composing, matching, and rewriting only the drone part of the model. Therefore, we continuously:

• Reload /drone and /var from the CDO repository.
• Compose them into a full bigraph.
• Look for matches of the rule.
• If conditionalRule is satisfied, and we haven’t just matched (canMatchAgain), apply the conditional rule.
• Update /composed and overwrite only the drone part in /drone.

AbstractBigraphMatcher<PureBigraph> matcher =
        AbstractBigraphMatcher.create(PureBigraph.class);

while (true) {
    Thread.sleep(100);

    // 1) Reload both parts
    PureBigraph bVarPart   = toBigraph(MM, template.find(cdoIdVar,   EObject.class, "/var"),   sig());
    PureBigraph bDronePart = toBigraph(MM, template.find(cdoIdDrone, EObject.class, "/drone"), sig());

    // 2) Compose into full model
    PureBigraph fullModel = merge(bDronePart, bVarPart, true);

    // 3) Find matches for the non-conditional rule
    Iterator<BigraphMatch<PureBigraph>> iterator =
            matcher.match(fullModel, low).iterator();

    if (iterator.hasNext()) {
        BigraphMatch<PureBigraph> match = iterator.next();

        // 4) Check attribute-based condition and throttle via canMatchAgain
        if (conditionalRule.isMatchValid(bVarPart, conditionalRule, match)
                && canMatchAgain.get()) {

            System.out.println("Value is low!");
            canMatchAgain.set(false);

            // 5) Apply reaction and persist results
            PureBigraph result =
                    new PureReactiveSystem().buildParametricReaction(fullModel, match, low);

            // Update composed model
            template.insert(result.getInstanceModel(), "/composed");

            // Decompose to get updated drone part only and persist it
            PureBigraph dronePartDecomposed = decompose(result, matcher);
            EObject inserted = template.insert(dronePartDecomposed.getInstanceModel(), "/drone");
            cdoIdDrone = CDOUtil.getCDOObject(inserted).cdoID();
        }
    }
}

A few important points:

• Only attributes of /var change; its structure stays the same. Therefore, we do not need to update the CDO ID or model for /var.
• The structural change (switching EnergyD.HIGH to EnergyD.LOW) is contained in the drone part, which is decomposed from the full result and overwrite in /drone.
• /composed always holds the latest composed state of the system.

The helper method decompose() uses a context-extraction rule (getDroneContextRule) and some bigraph algebra to recover a drone-specific part from the complete model.

Screenshots

Before	After

Executing the Test

Run the main java class Application.java from your IDE or directly from the terminal:

# (3. Terminal)
$ mvn spring-boot:run

Before, make sure that the rosbridge websocket server and the CDO database is running:

# (1. Terminal)
# Rosbridge server
$ ros2 launch rosbridge_server rosbridge_websocket_launch.xml

# (2. Terminal)
# CDO
$ mvn test -DrunDisabledTests=true -Dtest="CDOStandaloneServerTest#run_server_test_01"

Then publish test messages on the ROS 2 topic:

# A sufficient low value
$ ros2 topic pub /echo std_msgs/Int16 "{data: 123}"

# A sufficient high value
$ ros2 topic pub /echo std_msgs/Int16 "{data: 2000}"

Summary

You have now:

• Connected ROS2 to your Java bigraph application and streamed data into bigraph node attributes.
• Persisted a split bigraph using Spring Data CDO, modifying only the relevant parts over time.
• Implemented a conditional bigraph reaction rule that inspects attributes, and used the simulation API to match and rewrite the model incrementally.

Note

If the websocket server is not running or the application cannot connect, you will instead see an error such as:

[rosbridge_websocket-1] [INFO] [1763100529.046125905] [rosbridge_websocket]: Client connected. 1 clients total.
[rosbridge_websocket-1] [INFO] [1763100529.070165211] [rosbridge_websocket]: [Client 429a644c-0ce9-4882-b68a-89695f101b0e] Subscribed to /echo

If the websocket server is not running or the application cannot reach it, you will see an error similar to:

[ERROR]: Could not create WebSocket: Connection failed.